1. Field of the Invention
The present invention relates to a photomultiplier tube, and more particularly to a photomultiplier tube wherein the sidewall of the photomultiplier tube envelope is made of a metal.
2. Description of the Prior Art
There has been proposed a box-shaped photomultiplier tube as shown in FIG. 1 comprising an evacuated envelope 1 (made entirely of glass) having a generally cylindrical, disc-shaped, transparent faceplate 3, a generally cylindrical sidewall 2, and a generally cylindrical, disc-shaped stem 4. The faceplate 3 is hermetically attached to one opening of the cylindrical sidewall 2. A photocathode 5 is formed on the interior surface of the transparent faceplate 3 using alkali metal vaporization techniques. The photocathode 5 provides photoelectrons in response to radiation incident thereon. The stem 4 is vacuum sealed to the lower opening of the cylindrical sidewall 2 e.g., by welding or heat-melt-bonding. Inside the envelope 1 is provided an electron multiplier assembly 8 comprising a plurality of dynodes. Each dynode is provided with a secondary electron emissive surface for multiplying the photoelectrons incident thereon.
As shown in FIG. 1, the stem 4 is formed from a generally cylindrical glass disc 4A. A plurality of stem leads 6 (only some of which are shown) extend through the glass disc 4A into the envelope 1 for supplying voltages to the dynodes and the photocathode 5.
In the center of the glass disc 4a is a heat sealed glass exhaust tube 7 protruding vertically downward. During manufacture of the photomultiplier tube and before being heat sealed, the glass exhaust tube 7 provides communication between the interior of the photomultiplier and an exhaust system (not shown). The exhaust system evacuates the envelope 1 via the glass exhaust tube 7, and then alkali metal vapor is introduced into the envelope 1 through the glass exhaust tube 7 for forming the photocathode 5. The glass exhaust tube 7 is unnecessary after production of the photomultiplier tube is complete, and so is severed at the final stage of photomultiplier tube manufacture by using a gas burner so as to be maximally shortened.
The cylindrical sidewall 2 of conventional photomultiplier tubes is heated to melting at a sealing portion 2a and vacuum sealed to the cylindrical disc-shaped stem 4 thereat. After the glass exhaust tube 7 is connected to the exhaust system, the envelope 1 is evacuated and then alkali metal vapor is introduced into the envelope 1 to form the photocathode 5 and the secondary electron emissive surface of the dynodes. Afterward, the glass exhaust tube 7 is severed from the exhaust system using a gas burner and maximally shortened. Refer to Japanese Laid-open Patent Publications 60-112224, 58-54539, and 60-211758 for more detailed information on photomultiplier tube technology.
In view of the fact that the cylindrical sidewall 2 and the stem 4 of the photomultiplier tube are formed entirely from glass, various problems have been known with conventional photomultiplier tubes.
Firstly, light emanates from the glass caused by radioactive materials such as K.sup.40 contained within the glass and causes production of noise.
Secondly, floating electrons or ions generated during production of the photomultiplier assembly 8 strike the glass of the cylindrical sidewall or the stem and cause the glass to emit light which also produces unwanted noise.
Thirdly, the photomultiplier assembly 8 is liable to deteriorate because of a high temperature applied thereto when the stem 4 is melted to secure to the opening of the cylindrical sidewall 2 and when the glass exhaust tube 7 is severed by a gas burner.
Fourthly, melting and severing of the glass exhaust tube 7 cause generation and pooling of gas at the interior section of the photomultiplier tube, which in turn prevents forming a good vacuum. Further, severing of the glass exhaust tube 7 requires more than one step which prolongs the manufacturing time.
Finally, changes in heat, especially at the stem 4, brought about when glass is heated for severing the glass exhaust tube 7, generates cracks in the glass, shifts in the alkali metal film, and other undesirable phenomena, which complicate severing and shortening operations of the glass exhaust tube 7.